This invention relates to a production line coating system for coating a series of articles with particles.
Coating systems are known that have varying degrees of effectiveness. With a fluidized bed system a preheated article is agitated within a mass of the coating powder. Agitation of the articles is necessary to insure that the coating powder enters and adheres in recesses in the surface of the article. The minimum coating thickness is usually 10 mils and accurate temperature control is required to maintain uniform thickness among the articles. With a large article, a thickness gradient may be present, resulting in a heavier coating on the bottom than on the top, whereas a small article is difficult to coat.
Electrostatic fluidized beds usually require voltages ranging from 60 to 90 kilovolts, which present safety problems. Electrostatic shielding due to charge build up at sharp edges of articles, makes such beds ill suited for workpieces with small recesses.
Flocking guns are known, but require that the articles be preheated. Additionally, the coating uniformity is largely dependent on the skill of the operator. With these, as well, there is difficulty getting powder into recesses. Obtaining a uniform coating, and preventing a large overspray are further problems.
Electrostatic spray guns similarly require high voltages and are usually quite expensive. Again the uniformity of coating depends on the skill of the operator, and the electrostatic shielding effect makes it difficult to coat within deep recesses.
Electrogasdynamic coating is described in U.S. Pat. No. 3,673,463, granted to one of the present inventors. That disclosure is not specifically directed to the particular problems of production line coating.